1. Field of the Invention
This invention is related to magnetic disk drives and more specifically to a method for calibrating the glide sliders used in the manufacture of magnetic recording disks.
2. Description of the Background Art
Conventional magnetic disk drives are information storage devices which utilize at least one rotatable magnetic disk with concentric data tracks, a read/write recording head for reading and writing data on the various tracks on the disk, an air bearing slider in a generally flying mode for holding the recording head adjacent to the disk, a suspension for resiliently holding the slider and the recording head over the data tracks, and a positioning actuator connected to the suspension for moving the recording head across the disk to the desired data track and maintain the recording head over the data track during a read or a write operation.
The primary magnetic components in a magnetic disk drive are the recording head and the magnetic disk upon which information is recorded and subsequently retrieved. The recording head is attached to a slider which floats or flys on a cushion of air above the disk surface. In order to achieve high magnetic recording density it is necessary for the recording head to fly very close to the disk. There is also a requirement that no asperities or protuberances project from the disk surface and contact the slider. Accordingly, during the manufacturing of the disk, there is a need to accurately detect the presence of asperities, if any. This is generally referred to as glide height testing of the disk.
Typically the sliders used for glide height testing are called glide sliders and have an air bearing surface designed such that the flying height varies approximately linearly with respect to the relative speed of the disk (i.e. the speed between the disk and the slider). Because of manufacturing tolerances, glide sliders which have the same targeted design flying height will experience variation in the actual flying heights. Therefore it is necessary to calibrate the flying height of each glide slider. A common method of calibrating the flying height of a glide slider is to fly the slider on a transparent glass disk and measure the flying height using interferometry performed through the transparent disk. A problem with this method of calibration is that a smooth glass disk typically has a different surface topography compared to a magnetic disk which has a surface of lubricant and sputtered carbon. This difference in surface topography can result in a different flying height of the same slider depending on which disk is used. This effect is more pronounced with low flying heights. Contemporary flying heights of ten to twelve nanometers are substantially lower than just a few years ago. As flying heights become even lower in the future, the problems associated with using a transparent glass disk to calibrate glide testing heads will become more pronounced.
The interferometry calibration technique also suffers from mechanical mount tolerances. The mechanical mount tolerances are the variations in distance between the disk surface and the portion of the actuator upon which the suspension is mounted. The difference between the mechanical mount spacing of the interferometry tester and the disk glide tester may result in a difference in flying height of three to four nanometers. This is a very large fraction out of a total flying height of, for example, twelve nanometers. The difference in flying heights caused by mechanical mount tolerances could be eliminated if the glide slider could be calibrated in situ on the actual glide tester to be used for disk testing.
Finally the roll of the slider may be different between the interferometry calibration and the disk glide tester. Slider roll is when one corner of the slider dips below the average flying height of the slider. Roll is used here in the same sense as in aviation where in discussing airplane motion one wing rotates down and the other wing rotates up during a roll. In glide height testing it is the lowest point of the slider, the portion of the slider closest to the disk, which is most likely to first touch a bump or asperity. Therefore it is important to calibrate the glide height slider with respect to the point on the slider having the lowest flying height.
What is needed is a method of calibrating glide height sliders which is free from effects of disk surface topography, free from effects of mounting tolerances, and takes into account the roll of the slider.
In one embodiment of the present invention a glide slider is calibrated in situ on a disk glide tester which will subsequently be used for disk testing. One embodiment is to use a disk which has fabricated bumps with calibrated heights. A verification is first made to insure that observed contacts are between the glide slider and the bumps. Then the flying height of the glide slider is then raised above the bumps and then gradually reduced by decreasing the speed between the glide slider and the disk until contact occurs. In this manner the differences in flying height from mounting on different testers is solved by calibrating the glide slider on the tester which is to be used in manufacturing for testing disks. Also the surface topography is dominated by lubricant and sputtered carbon for both the disk with bumps and the disks to be tested. Accordingly there is no significant difference in flying heights generated by different surface topographies.
In another embodiment of the present invention, the glide slider is dynamically scanned over the calibrated bumps. Dynamic scanning is accomplished by moving the glide slider radially over the disk at the approximate radial location of the calibrated bump. This insures that the roll of the glide slider is taken into account during the fly height calibration and that the lowest portion of the glide slider is the portion that is in contact with the bump.
Other aspects and advantages of the present invention will become apparent from the following detailed description, which along with the accompanying drawings illustrate by way of example the principles of the invention.